A conventional outer rotor-type DC brushless motor will be explained with reference to FIGS. 3A-4.
Firstly, a structure of a stator 50 will be explained with reference to FIG. 3A. An oil-impregnated sintered bearing 52, which is formed into a cylindrical shape, is coaxially provided in a center hole 51a of a cylindrical housing 51. A stator core 53, in which a plurality of pole teeth 53a are radially outwardly extended and coils 54 are respectively formed on the pole teeth 53a, is attached on an outer circumferential face of one end part of the housing 51. In the stator core 53, each of the coils 54 is formed by winding a copper wire on each of the pole teeth 53a coated with and electrically insulated by a resin film. A mounting plate 55 is fixed on the outer circumferential face of the other end part of the housing 51 by press fit or caulking. A motor driving circuit board 56 is provided on the mounting plate 55. A sensor (not shown) including a Hall element is mounted on the mounting plate 56.
Next, a structure of a rotor 57 will be explained with reference to FIG. 3A. One end part of a rotor shaft 58 is fixed to a rotor hub 59, which is formed by pressing a metal plate, e.g., brass plate, by press fit. A rotor yoke 60, which is formed into a cup shape, is integrally attached to the rotor hub 59 by caulking. A ring magnet 61, in which a plurality of magnetic poles are arranged in the circumferential direction, is fixed on an inner circumferential face of the rotor yoke 60. After assembling the stator 50, the rotor shaft 58 of the rotor 57 is inserted into the oil-impregnated sintered bearing 52. The pole teeth 53a and the magnet 61 are faced each other. Since the rotor shaft 58 is rotatably supported by the oil-impregnated sintered bearing 52, the rotor 57 can be rotated with respect to the housing 51.
Since the rotor shaft 58 of the rotor 57 is supported by the oil-impregnated sintered bearing 52 incorporated in the center hole 51a of the housing 51, a vertical load of the rotor 57 is received by a pair of metal or resin washers, or a combination of a metal washer and a resin washer, which are provided between the rotor hub 59 and the oil-impregnated sintered bearing 52.
For example, as shown in FIG. 3B, a first resin washer 62 is provided on an end face of the rotor hub 59, and a second resin washer 63 is provided on an end face of the oil-impregnated sintered bearing 52. With this structure, directly sliding the end face of the rotor hub 59 on the end face of the oil-impregnated sintered bearing 52 can be avoided. The first resin washer 62 and the second resin washer 63 are ring-shaped resin washers having high slidability and high abrasion resistance. Therefore, the first resin washer 62 and the second resin washer 63 slide on each other while rotating the rotor 57, so that abrasion of the oil-impregnated sintered bearing 52 can be prevented (see Japanese Laid-open Patent Publications No. 7-15907, No. 2007-247663 and No. 2008-312286).
However, the first resin washer 62 shown in FIG. 3B is press-fitted to the rotor shaft 58 and placed on the lower end face of the rotor hub 58. On the other hand, the second resin washer 63, whose outer diameter is smaller than an inner diameter of the center hole of the housing 51, is inserted in the center hole and placed on the upper end face of the oil-impregnated sintered bearing 52.
With this structure, in some cases, the first resin washer 62 is rotated together with the second resin washer 63 by the rotation of the rotor 57. Especially, the first resin washer 62 and the second resin washer 63 will be stuck on each other by surface tension of oil included in the oil-impregnated sintered bearing 52, so the first resin washer 62 and the second resin washer 63 will be easily integrally rotated. By integrally rotating the first resin washer 62 and the second resin washer 63, the second resin washer 63 slides on the end face of the oil-impregnated sintered bearing 52, and an outer circumferential face of the second resin washer 63 slides on an inner circumferential face of the housing 51. Therefore, sludge or abrasion powders, which are formed by the sliding actions, invade into the center hole of the housing 51, so the rotor shaft 58 will be abraded and decentered and a span of life of the oil-impregnated sintered bearing 52 will be shortened.
Note that, in the conventional technology disclosed in said Japanese Laid-open Patent Publications No. 7-15907, No. 2007-247663 and No. 2008-312286, two or more washers are stuck, on the bearing, in the thrust direction. Therefore, number of parts must be increased, a production cost must be increased and assembling efficiency must be lowered.
In case that the second resin washer 63 is a ring-shaped washer having two projected sections 63a and that concave sections 51a facing the projected sections 63a are formed in an inner wall of the housing 51 as shown in FIG. 4, the integrated rotation of the first and second ring washers 62 and 63 can be prevented. However, the inner wall of the housing 51 must be cut, so a production cost must be increased.
Further, the second resin washer 63 is formed into the non-circular shape. Therefore, in case that the second resin washer 63 is detached from the center hole of the housing 51 by surface tension of the lubricant oil when the rotor 57 is lifted in the axial direction by an external load or a treatment in an examination step, the second resin washer 63 cannot always correctly fitted into the center hole of the housing 51. Therefore, there is a possibility that the projected sections 63a of the second resin washer 63 are not fitted into the concave sections 51a of the housing 51. If the projected sections 63a are not fitted into the concave sections 51a, abrasion of the bearing 52 cannot be prevented.